The McMurray oil sands of Alberta constitute one of the largest deposits of hydrocarbons in the world.
At the present time, there are two very large scale plants extracting bitumen (a heavy and viscous oil) from these oil sands. Each of these plants incorporates a sequence of mining, bitumen extraction and bitumen upgrading operations.
For many years, the as-mined oil sand was moved by conveyor belt assemblies from the mine site to the extraction plant.
In recent years, slurry pipelines have begun to replace the conveyor belt systems.
In connection therewith, one needs to provide suitable means for slurrying the oil sand with water and entrained air, to produce a slurry that is suitable for pumping down the pipeline.
The mixer circuit so provided is required to cope with very large volumes of throughput--typically 10,000 tons of oil sand per hour. The oil sand is highly erosive, so the mixer circuit should have minimal moving parts and be very durable. In addition, the as-mined oil sand contains a variety of lumps including rocks, clay lumps, and oil sand lumps. The concentration of lumps is greater in winter, when some of the oil sand reports in the form of frozen chunks. Usually the as-mined oil sand will have passed through a double roll crusher prior to slurrying, to reduce lump size below 24 inches. However, the crushed oil sand still contains oversize lumps which are unsuitable for pumping and feeding into the pipeline. Therefore the mixer circuit requires some means for rejecting the oversize lumps (otherwise referred to herein as "oversize").
In U.S. Pat. No. 5,039,227, issued to Leung et al and assigned to the owners of the present application, one mixer circuit for this purpose has been disclosed.
In the Leung et al mixer circuit, an oil sand stream is dropped from the end of a conveyor into a mixer tank. The mixer tank is open-topped, has a cylindrical body and conical bottom and forms a central bottom outlet. A swirling vortex of slurry is maintained in the tank and the incoming oil sand is fed into it. Slurry leaves the tank through the bottom outlet, is screened using vibrating screens to reject oversize, and is temporarily collected in an underlying pump box. Some of the slurry in the pump box is withdrawn and pumped back through a return line to be introduced tangentially into the mixer tank to form the swirling vortex. The balance of slurry in the pump box is withdrawn and pumped into the pipeline.
The Leung et al mixer circuit has been successfully applied on a commercial scale. However, it is characterized by certain shortcomings.
One problem has to do with the fact that a large proportion of the produced slurry has to be pumped back into the mixer tank to maintain the vortex. As a result, the slurry volume that undergoes screening is about twice the volume pumped into the pipeline. This requires provision of a very large screen area. The screens are necessarily located in confined quarters. As a result, one cannot increase the throughput of the circuit because the screens constitute a bottleneck that is not easily resolved.
Another problem lies in the oversize reject rate. At present the screens reject lumps having a diameter greater than 2 inches. About 10% of the oil sand feed is so rejected. These rejects represent a significant oil loss. To reduce this loss, the rejects are conveyed to a second mixer circuit and are re-processed. This is expensive to implement.
From the foregoing, it is apparent that there is a need for a mixer circuit which operates without slurry recycle and which has improved reject rates.
It is the purpose of the present Invention to provide such a mixer circuit.